In future cellular 5G systems new spectrum at significantly higher carrier frequencies will be used. This introduces a number of problems. First of all, the reference dipole antenna area is reduced, which reduces range and coverage as compared to present cellular technology. Moreover, new radio frequency electromagnetic field requirements limits the transmit power to less than 13 dB above 6 GHz, which in turn reduces range and coverage as compared to present cellular systems. Another problem relates to the fact that for higher carrier frequencies certain radio propagation effects, in particular diffraction, i.e. shadowing, and obstacle penetration losses increases. These increased losses will in turn reduce the range and coverage as compared to present cellular systems.
The above challenges and others require a shift towards quite massive beamforming. This is the one foreseen solution that provide the antenna gain needed to reach a sufficient range and coverage. Beamforming is a technique to form the antenna gain of an antenna array by application of suitable phase shifts to each of the antenna elements of the array. In that way, constructive combination of the radio wave wave-fronts of each antenna elements tends to amplify the signal in selected directions. Similarly destructive combination tends to reduce the effective signal in other directions.
Beamforming can in general be performed on both analogue signals as well as digital signals. In case of analogue beamforming, the analogue transmit signal is sent to a set of antenna elements, one set for each Multiple Input Multiple Output layer, MIMO layer. The phase shift of each antenna in a set is then controlled in a way to create the radio beam.
The advantage of analogue beamforming is that a digital to analogue conversion does require a number of AD/DA converters that is equal to the number of spatial multiplexing layers. This number is likely to be much lower than the number of antenna elements, since the majority of the degrees of freedom are to be used for beamforming. The downside is a lack of flexibility in that all resource blocks, for example the case of OFDM type multiple access as in the LTE system, are subject to the same beamforming. This is only consistent with the scheduling of users in a single direction per beam, at each time instant and for all frequencies of the band.
In case of digital beamforming each antenna element is equipped with a separate AD/DA converter. This allows beamforming weights to be added in base band, rather than in the analogue domain. The advantage of this solution in turn is flexibility, since each user can be given a separate beamforming at the same time. It is therefore no longer necessary to limit the scheduling to users in a single direction. The drawback is instead that there is a need for a large number of AD/DA converters. This leads to extensive costs and to a large power consumption.
As stated above, a low cost analogue beamformer uses the same beam pattern for the whole radio frequency band, at a given point in time. Since the beamformed antenna gain is highly directional and since wireless devices such as User Equipments, UEs, are located in individual directions as counted from a base station, only one or a few UEs can be communicated with in a given point in time. This means that the antenna patterns need to be changed over time to direct the power to each UE in a cell.
Some further drawbacks with the known beamforming technology include the fact that in the case that a low power and low cost beamforming solutions are sought, then the flexibility with digital beamforming lost. The analogue beamforming on the other hand need to be able to handle multiple beams per users, to capture reflected energy at higher carrier frequencies. This furthermore requires complex recalculations of phase shifts and reduces the antenna gain in each direction. Alternatively, very large phase shifting tables are needed. There is also a need to continuously search for new beam directions and to initiate tracking of such beams. This is complicated by the directional properties of the 5G radio propagation, at high carrier frequencies.
The beamforming technology to be utilized also need to reflect the fact that the propagation in general becomes more beamlike when the carrier frequency increases. As a direct consequence of this, there is only a few directions available at a time for beamformed based communication between two static, i.e. non-moving, communication devices since the beam transmitting device only can reach the receiving device with a beam if the receiving device lies within cross-section area of the beam. This is further complicated by the fact that wireless devices may move within the cells. A relative movement between the communicating devices will render the initially available directions unusable. The initially available directions may also become unusable even for static communication devices, such as two radio base stations, if there are changes in the environment. A particular example may be that a house or some other construction is raised between the radio base station thereby blocking the initially available transmit direction. This particular drawback with beamformed transmissions needs to be addressed to obtain an efficient use of the high carrier frequencies utilized in, for example, 5G-network technology.
The proposed technology aims to at least partially overcome the mentioned drawbacks of the prior art solutions.
One suggestion for handling beamforming is provided by REF. [1], REF. [1] discloses bidirectional iterative beam forming techniques. An apparatus such as a wireless device having an antenna control module is operative to initiate beam formation operations using an iterative training scheme to form a pair of communications channels for a wireless network.